Field of the Invention
The present invention relates to methods and systems for improving surgical procedures for correcting astigmatism.
Discussion of Related Art
A common method of surgically correcting astigmatism is the method of forming limbal relaxing incisions (LRIs) in the eye. As shown in FIG. 1, such LRIs 100 are generally paired arcuate incisions/cuts formed in the cornea 102 of the eye 104, wherein the LRIs 100 subtend an angle relative to a center 106 of the eye 104 that has a magnitude ranging from between about 20° to about 100°. In the example shown in FIG. 1, the angle is approximately 65°. The incisions 100 are typically formed with a diamond or other blade such that they have a depth that is generally from 80% to 100% of the thickness of the cornea 102 and are within around 0.5 mm to 2 mm of the limbus of the eye. The paired arcuate incisions 100 are diametrically opposed across the cornea 102 and disposed along a steep axis of the cornea. The incisions are made at an angle relative to the scale shown at the perimeter of the eye. This “clock” angle, 20° in FIG. 1, is along the direction of the steep axis of the astigmatism. The other indicated angle, 65° in this case, is the subtended arc angle, which is related to the magnitude of the astigmatism being treated. As shown in FIG. 1, the incisions 100 are bisected by the astigmatism axis 108. The standard depth of the incision is 90% of the thickness of the cornea near the limbus (or, in some cases, to a standard thickness of around 600 μm, which represents a depth of about 90% of an average corneal thickness near the limbus). The incisions can be formed either manually with a blade or automatically using a femtosecond laser. By femtosecond laser is meant a laser with a pulse width of between about 100 fs and 10,000 fs.
The mechanism by which the LRIs 100 reduce or eliminate astigmatism is mediated by the changes in the biomechanical structure of the cornea 102 caused by the incisions 100. In particular, the incisions 100 result in a change in the shape of the corneal surfaces such that there is a flattening of the curvature of the corneal surface along the axis connecting the paired arcuate incisions 100. The particular form and magnitude of curvature of the cornea 102 is a result of an equilibrium achieved between an outward force applied by the pressure inside the eye (intraocular pressure or IOP) and inward forces generated by the restoring force of the stressed collagen fibrils which make up the bulk of the cornea 102. The arcuate incisions 100 cut through the collagen fibrils resulting in a weakening of the cornea 100 in a direction perpendicular to the length of the incision. Such weakening allows for greater strain or lengthening of the intact fibrils just posterior to the incision and consequently results in the flattening of the curvature of the cornea perpendicular to the length of the incision.
Though LRIs are fairly widely used for correction of residual astigmatism, particularly for patients undergoing cataract surgery, the procedure is used for only a relatively small fraction of eligible patients (EyeNet Magazine, article 000506, American Academy of Ophthalmology; Nichamin et al, Cataract and Refractive Surgery Today, “Corneal Relaxing Incisions”, August, 2009,). One reason that the procedure is not more universally utilized is that the results of the procedure in correcting astigmatism are variable (Mingo-Botin et al, Journal of Cataract & Refractive Surgery
Volume 36, Issue 10, Pages 1700-1708, October 2010; Walter Bethke, Review of Ophthalmology, March 2011). The source of variability of the results, though not fully understood, is likely due in part to several factors, such as: 1) variation in the depth or shape of the incisions (due to limits of dexterity of the surgeon, etc.), 2) patient-to-patient variability in the pattern or arrangement of collagen fibrils in the cornea (causing identical incisions to have different effects for different patients), and 3) long term corneal health being compromised by incisions that cut nearly or completely through the cornea.
Recently, the practice of making the incisions manually with a fixed or variable depth blade is starting to be supplanted by incisions made with a femtosecond laser (Maxine Lipner, EyeWorld, “What's Ahead, Femtosecond technology changing the cataract landscape”, 2011 Mar. 24 8:45:27). Such a laser makes incisions by focusing ultrashort laser pulses to a very fine focus, causing a plasma mediated photodisruption of the tissue at the point of focus. An incision is generated by placing a contiguous series of such pulses in a pattern that results in the formation of the desired incision. The combined effect of the pattern of pulses is to cleave the tissue at the targeted plane. Arbitrarily complex incisions patterns can be generated with such lasers. The femtosecond lasers are believed to make incisions of a more accurate and consistent depth and of a curvature that more accurately matches the desired arcuate form of the incision. While use of such a laser addresses the first of the concerns mentioned previously, i.e., variability in the clinical outcomes of LRIs due to imprecise cuts, such uses did not address the other two concerns, i.e., patient-to-patient variability in the pattern or arrangement of collagen fibrils in the cornea, and the comprising of long term corneal health by deep incisions.